A light-modulating liquid-crystal element comprising liquid crystals and a supporting transparent solid substance (e.g. a polymer) becomes cloudy (or white turbid) in the absence of an electric field, because incident light is scattered due to a difference in refractive indices between the transparent solid substance (e.g. a polymer) and liquid crystals, and other factors. On the other hand, when an electric field is applied, the liquid crystals align in the direction of the electric field and the refractive indices of the transparent solid substance (e.g. a polymer) and liquid crystals become similar, resulting in a transparent state. When the electric field is turned off, the element returns to the original cloudy state.
The characteristics required of a light-modulating liquid-crystal element are (1) that it shows sufficient contrast between the transparent state and the white turbid state, (2) that it can be driven at a low voltage, and (3) that it can operate in a wide temperature range, and the like. Furthermore, because the light-modulating liquid-crystal element is often used as a construction material and the like, there is a problem of yellowing by sunlight, room-light, and the like. Therefore, it is also an important requirement (4) that the light-modulating element has superior light resistance (or weather resistance).
To date, many kinds of light-modulating liquid-crystal elements have been proposed in order to satisfy these requirements.
For example, Patent Document 1 discloses a light-modulating liquid-crystal element wherein capsules filled with a nematic liquid-crystal material are dispersed in a resin. However, the process for manufacturing this light-modulating liquid-crystal element is complicated because of the presence of a microencapsulation step. Moreover, because water soluble polymers or aqueous polymer emulsions are used to prepare the element, its water resistance is inferior and, as a result, shows defects such as white turbidity, swelling, and deterioration of physical properties.
Further, Patent Documents 2 and 3 disclose light-modulating elements, wherein the liquid crystals that are initially dissolved in a resin are dispersed as microdroplets in the resin matrix as a result of phase separation of the liquid crystals during solidification of the resin. However, the ranges of applicable temperature of these light-modulating elements were narrow and their light resistance (or weather resistance) was unsatisfactory.
Further, Patent Document 4 discloses a light-modulating element wherein a nematic liquid-crystal material forms a continuous layer, in which is dispersed a transparent solid substance as a three-dimensional network. However, this light-modulating element was insufficient in the contrast between the transparent state and cloudy (or white turbid) state, and further was unsatisfactory with respect to the light resistance (or weather resistance).
Further, Patent Document 5 discloses a light-modulating element which comprises liquid crystals of a specific structure (i.e. tolans). However, this light-modulating element was inferior in light resistance (or weather resistance) and had a problem of yellowing.
As described above, the light-modulating elements that have been proposed to date did not fully satisfy the aforementioned requirements and the prevention of yellowing was insufficient in particular. Thus, there was a significant problem with the light resistance (or weather resistance).
Patent Document 1: Japanese Patent Laid-Open Publication No. H3-52843
Patent Document 2: Japanese Patent No. 2721497
Patent Document 3: Japanese Patent No. 2614854
Patent Document 4: U.S. Pat. No. 5,374,371
Patent Document 5: Japanese Patent No. 3427905